Recently, in view of sharp rise of crude oil price since oil crisis, demands for versatility of sources of energy or the like, the drilling for oil and natural gas and the pipeline construction in a very cold land such as the North Sea, Canada and Alaska have been actively promoted. Further, the development of a sour gas field and the like whose development was once abandoned because of its strong corrosion has also recently been developed vigorously.
Further, here, with respect to a pipeline, there has been observed a trend where a transport operation is performed using a large-diameter pipe under a high pressure to enhance transport efficiency of natural gas or oil. To withstand a high-pressure operation in a pipeline, it is advantageous to form a transport pipe (line pipe) using a heavy wall thickness pipe so that a UOE steel pipe which is formed of a plate is used. Recently, however, there have been strong demands for the further reduction of construction cost of a pipeline or demands for the reduction of a material cost of steel pipes due to the unstable supply sufficiency of UOE steel pipes. Accordingly, as a transport pipe, in place of a UOE steel pipe which uses a plate as a raw material, a high strength electric resistance welded steel pipe or a high strength spiral steel pipe which is formed using a coil-shaped hot-rolled steel sheet (hot-rolled steel strip) which possesses high productivity and can be produced at a lower cost has been used.
These high strength steel pipes are required to possess excellent low-temperature toughness from a viewpoint of preventing bust-up of a line pipe. To manufacture such a steel pipe which possesses both of high strength and high toughness, attempts have been made to impart higher strength to a steel sheet which is a raw material of a steel pipe by transformation strengthening which makes use of accelerated cooling after hot rolling, precipitation strengthening which makes use of precipitates of alloy elements such as Nb, V, Ti or the like, and attempts have been made to impart higher toughness to the steel sheet through the formation of microstructure by making use of controlled rolling or the like.
Further, a line pipe which is used for transporting crude oil or natural gas which contains hydrogen sulfide is required to be excellent in so-called sour gas resistances such as hydrogen induced cracking resistance (HIC resistance), or stress corrosion cracking resistance in addition to properties such as high strength and high toughness.
To satisfy such a demand, patent document 1, for example, proposes a method of manufacturing a low yield ratio and high strength hot rolled steel sheet which possesses excellent toughness, wherein steel which contains 0.005 to 0.030% or less C and 0.0002 to 0.0100% B, and contains 0.20% or less Ti and 0.25% or less Nb in a state where either or both of Ti and Nb satisfy the relationship of (Ti+Nb/2)/C: 4 or more, and further contains proper amounts of Si, Mn, P, Al and N is subjected to hot rolling and, thereafter, is cooled at a cooling rate of 5 to 20° C./s, and is coiled at a temperature range from more than 550° C. to 700° C. thus manufacturing the hot rolled steel sheet in which the structure is formed of ferrite and/or bainitic ferrite, and an amount of solid solution carbon in grains is set to 1.0 to 4.0 ppm. According to the technique disclosed in patent document 1, it may be possible to manufacture a high strength hot rolled steel sheet which possesses excellent toughness, excellent weldability and excellent sour gas resistance, and also possesses a low yield ratio without causing non-uniformity of a material in the thickness direction as well as in the length direction.
However, in the technique disclosed in patent document 1, the amount of solid solution carbon in grains is 1.0 to 4.0 ppm and hence, due to charged heat at the time of performing girth weld, the growth of crystal grains is liable to occur so that a welded heat affected zone becomes coarse grains thus giving rise to a drawback that toughness of the welded heat affected zone of the girth weld portion is easily deteriorated.
Further, patent document 2 proposes a method of manufacturing a high strength steel sheet which possesses excellent hydrogen induced cracking resistance, wherein a steel slab which contains 0.01 to 0.12% C, 0.5% or less Si, 0.5 to 1.8% Mn, 0.010 to 0.030% Ti, 0.01 to 0.05% Nb, 0.0005 to 0.0050% Ca such that 0.40 or less of carbon equivalent and 1.5 to 2.0 Ca/O are satisfied is subjected to hot rolling at a temperature of Ar3+100° C. or more and, thereafter, the steel strip is subjected to air cooling for 1 to 20 seconds. Then, the steel strip is cooled down from a temperature not below the Ar3 point, the steel strip is cooled to a temperature of 550 to 650° C. within 20 seconds and, thereafter, the steel strip is coiled at a temperature of 450 to 500° C. According to the technique disclosed in the patent document 2, a line-pipe-use steel sheet of a grade X60 to X70 in accordance with the API standard having hydrogen induced cracking resistance can be manufactured. However, the technique disclosed in patent document 2 cannot secure a desired cooling time when it comes to a steel sheet having a large thickness thus giving rise to a drawback that it is necessary to further enhance cooling ability to secure desired characteristics.
Patent document 3 proposes a method of manufacturing a high strength line-pipe-use plate which possesses excellent hydrogen induced cracking resistance, wherein steel containing 0.03 to 0.06% C, 0.01 to 0.5% Si, 0.8 to 1.5% Mn, 0.0015% or less S, 0.08% or less Al, 0.001 to 0.005% Ca, 0.0030% or less O in a state where Ca, S, and O satisfy a particular relationship is heated, the steel is subjected to accelerated cooling from a temperature of an Ar3 transformation point or more to 400 to 600° C. at a cooling rate of 5° C./s or more and, immediately thereafter, the steel is reheated to a plate surface temperature of 600° C. or more and a plate-thickness-center-portion temperature of 550 to 700° C. at a temperature elevation speed of 0.5° C./s or more thus setting the temperature difference between the plate surface temperature and the plate-thickness-center-portion temperature at a point of time that reheating is completed is set to 20° C. or more. According to the technique disclosed in patent document 3, it is possible to obtain a plate where a structural fraction of a secondary phase in the metal structure is 3% or less, and the difference in hardness between a surface layer and a plate thickness center portion is within 40 points at Vickers hardness thus providing a plate possessing excellent hydrogen induced crack resistance. However, the technique disclosed in patent document 3 requires a reheating step thus giving rise to drawbacks that a manufacturing process becomes complicated, and it is necessary to further provide reheating equipment or the like.
Further, patent document 4 proposes a method of manufacturing steel material having a coarse-grained ferrite layer on front and back surfaces thereof, wherein a slab containing 0.01 to 0.3% C, 0.6% or less Si, 0.2 to 2.0% Mn, 0.06% or less P, S, Al, 0.005 to 0.035% Ti, 0.001 to 0.006% N is subjected to hot rolling, the slab is subjected to rolling at a temperature of Ac1−50° C. or below with cumulative rolling reduction of 2% or more in a cooling step which follows hot rolling and, thereafter, the slab is heated to a temperature above Ac1 and below Ac3, and is gradually cooled. The technique disclosed in patent document 4 is considered to contribute to the enhancement of SCC sensibility (stress corrosion cracking sensibility), weather resistance and corrosion resistance of a plate and, further, the suppression of deterioration of quality of material after cold working and the like. However, the technique disclosed in patent document 4 requires a reheating step thus giving rise to drawbacks that a manufacturing process becomes complicated, and that it is necessary to further provide reheating equipment or the like.
Further, recently, from a viewpoint of preventing burst rupture of a pipeline, it is often the case that a steel pipe for a very cold area is required to possess excellent toughness, and particularly, the excellent CTOD characteristics (crack tip opening displacement characteristics) and DWTT characteristics (drop weight tear test characteristics).
To satisfy such a requirement, for example, patent document 5 discloses a method of manufacturing a hot-rolled steel sheet for a high strength electric resistance welded steel pipe, wherein a slab which contains proper amounts of C, Si, Mn and N, contains Si and Mn to an extent that Mn/Si satisfies 5 to 8, and contains 0.01 to 0.1% Nb is heated and, thereafter, the slab is subjected to rough rolling under conditions where a reduction ratio of first rolling performed at a temperature of 1100° C. or more is 15 to 30%, a total reduction ratio at a temperature of 1000° C. or more is 60% or more and a reduction ratio in final rolling is 15 to 30% and, thereafter, the slab is cooled such that a temperature of a surface layer portion becomes a Ar1 point or below at a cooling rate of 5° C./s or more once and, thereafter, finish rolling is started at a point of time where the temperature of the surface layer portion becomes (Ac3−40° C.) to (Ac3+40° C.) due to recuperation or forced overheating, the finish rolling is completed under conditions where a total reduction ratio at a temperature of 950° C. or below is 60% or more and a rolling completion temperature is the Ar3 point or more, cooling is started within 2 seconds after completing the finish rolling, the slab is cooled to a temperature of 600° C. or below at a speed of 10° C./s, and the slab is coiled within a temperature range of 600° C. to 350° C. According to the steel sheet manufactured by the technique disclosed in patent document 5, it is unnecessary to add expensive alloy elements to the steel sheet, the structure of the surface layer of the steel sheet is made fine without applying heat treatment to the whole steel pipe thus realizing the manufacture of a high strength electric resistance welded steel pipe which possesses excellent low-temperature toughness, and particularly the excellent DWTT characteristics. However, with the technique disclosed in patent document 5, a steel sheet having a large sheet thickness cannot secure desired cooling rate thus giving rise to a drawback that the further enhancement of cooling ability is necessary to secure the desired property.
Further, patent document 6 discloses a method of manufacturing a hot rolled steel strip for a high strength electric resistance welded pipe which possesses excellent low-temperature toughness and excellent weldability, wherein a steel slab which contains proper amounts of C, Si, Mn, Al, N and also contains 0.001 to 0.1% Nb, 0.001 to 0.1% V, 0.001 to 0.1% Ti, also contains one or two kinds or more of Cu, Ni, Mo, and has a Pcm value of 0.17 or less is heated and, thereafter, finish rolling is completed under a condition where a surface temperature is (Ar3−50° C.) or more, and immediately after rolling, the rolled sheet is cooled, and the cooled rolled sheet is gradually cooled at a temperature of 700° C. or below while being coiled.
However, recently, a steel sheet for a high strength electric resistance welded steel pipe is required to further enhance low-temperature toughness, particularly the CTOD characteristics and the DWTT characteristics. With the technique disclosed in patent document 6, the low temperature toughness is not sufficient thus giving rise to a drawback that it is impossible to impart the excellent low-temperature toughness to the steel sheet for a high strength electric resistance welded steel pipe to an extent that the steel sheet sufficiently satisfies the required CTOD characteristics and DWTT characteristics.
Particularly, an extra thick hot rolled steel sheet having a sheet thickness exceeding 22 mm has tendency that cooling of a sheet thickness center portion is delayed compared to cooling of a surface layer portion so that a crystal grain size of the sheet thickness center portion is liable to become coarse thus giving rise to a drawback that the further enhancement of low temperature toughness is difficult.